Cryopumps currently available, whether cooled by open or closed cryogenic cycles, generally follow the same design concept. A low temperature array, usually operating in the range of 4 to 25 K, is the primary pumping surface. This surface is surrounded by a higher temperature radiation shield, usually operated in the temperature range of 70 to 130 K, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing which is closed except at a frontal array positioned between the primary pumping surface and a chamber to be evacuated. This higher temperature, first stage frontal array serves as a pumping site for higher boiling point gases such as water vapor.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shield and condense on the lower temperature array. A surface coated with an adsorbent such as charcoal or a molecular sieve operating at or below the temperature of the colder array may also be provided in this volume to remove the very low boiling point gases such as hydrogen. With the gases thus condensed and/or absorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
Once the high vacuum has been established, work pieces may be moved into and out of the work chamber through partially evacuated load locks. With each opening of the work chamber to the load lock, additional gases enter the work chamber. Those gases are then condensed into the cryopanels to again evacuate the chamber and provide the necessary low pressures for processing. After continued processing, perhaps over several weeks, gases condensed or adsorbed on the cryopanels would have a volume at ambient temperature and pressure which substantially exceeds the volume of the cryopump chamber. If the cryopump shuts down, that large volume of captured gases is released into the cryopump chamber. To avoid dangerously high pressures in the cryopump with the release of the captured gases a pressure relief valve is provided. Typically, the pressure relief valve is actuated when the cryopump chamber exceeds about 3 pounds per square inch gauge. Because the process gases may be toxic, the pressure relief valve is often enclosed within a housing which directs the gases through an exhaust conduit.
After several days or weeks of use, the gases which have condensed onto the cryopanels and, in particular, the gases which are adsorbed begin to saturate the system. A regeneration procedure must then be followed to warm the cryopump and thus release the gases and to remove the gases from the system. As the gases are released, the pressure in the cryopump increases and the gases are exhausted through the pressure relief valve.
A typical pressure relief valve includes a cap which when the valve is closed, is held against an elastomeric o-ring seal by a spring. With pressures which open the valve, the cap is pushed away from the o-ring seal and the exhausted gases flow past the seal. Along with the gas, debris resulting from processing within the work chamber also pass the seal. As this debris collects on the seal leaking past the seal may result.
For a cryopump operating at Ultra High Vacuum (UHV) pressures (10.sup.-9 Torr) or lower, conventional o-ring seals are not adequate to maintain the required leak integrity. Thus, valves having metal seals under very high closing force are used in the regeneration path. A metal sealed burst disc is additionally provided as the pressure relief device to satisfy safety requirements. However, this one-shot non-resealing device is expensive and requires replacement after a single use.